The invention relates to a spread spectrum receiver and particularly to stepwise decimation in a spread spectrum receiver.
In spread spectrum systems, the bandwidth used for transmitting a signal is substantially wider than is required for the data to be transmitted. The spectrum of a signal is spread in the transmitter by means of a pseudo-random spreading code, which is independent of the original data. In the receiver, a code replica, which is an identical copy of said spreading code, is used to narrow the spectrum of a signal. Spread spectrum systems can be coarsely divided into direct sequence (DS) spread spectrum systems and frequency hopping (FH) spread spectrum systems. In frequency hopping systems, the transmission frequency is varied in accordance with a pseudo-random spreading code within the limits of the available bandwidth, i.e. hopping occurs from one frequency to another. In direct sequence systems, the spectrum is spread to the available bandwidth by shifting the phase of the carrier in accordance with a pseudo-random spreading code. The bits of a spreading code are usually called chips as distinct from actual data bits.
FIG. 1A is a block diagram of a spread spectrum system based on a direct sequence, a transmitter 101 comprising not only a data modulator 104, but also a spreading code modulator 106 for spreading a transmitted spectrum by means of a spreading code. A receiver 102 comprises a despreading modulator 108, which operates with a spreading code replica identical to said spreading code and correlates a received signal with said spreading code replica. If the spreading code and the spreading code replica generated in the receiver are identical, and the spreading code replica and the spreading code included in the received signal are in phase, a data modulated signal preceding the spreading is obtained from the output of the despreading modulator 108. At the same time, any spurious signals are spread. A filter 110, which succeeds the despreading modulator 108, lets the data modulated signal through, but removes most of the power of a spurious signal, which improves the signal-to-noise ratio of the received signal.
FIG. 1B shows a prior art spread spectrum receiver. A received signal SRF is mixed by multipliers 112 and 114 with a sine-phased and cosine-phased component generated by a local oscillator 116, and filtered with low-pass filters 118 and 120 to generate intermediate-frequency I_if (in-phase) and Q_if (quadrature) signals. The I_if and Q_if signals are then subjected to analog-to-digital conversion in A/D converters 122 and 124, and applied to a digital receiver part 126, in which code and carrier demodulation is performed, and whose output is further connected to a data demodulator (not shown) which performs data demodulation to the signal.
FIGS. 2A and 2B are block diagrams of two such prior art implementations of the digital receiver part of a spread spectrum receiver based on direct sequence spreading that are usable as the digital receiver part 126 of FIG. 1B. The double lines in the block diagrams denote I and Q signals. In the implementation of FIG. 2A, an incoming intermediate-frequency signal Sin is first multiplied by a local carrier replica generated in a frequency generator 203 using a carrier mixer 202 to remove the carrier and the Doppler shift, whereupon it is multiplied in a code mixer 204 by a local spreading code replica generated by a code generator 207 controlled by a frequency generator 205. The multiplication by the spreading code replica provides despreading and narrows the spectrum of the signal. Next, the narrowband signal obtained from the code mixer 204 is filtered with a low-pass filter 206 to remove noise and interference, and the sampling frequency of the low-pass filtered signal is lowered to a frequency according to the spectrum of the data modulation with a decimator 208. Signal Sout obtained from the decimator 208 is applied to carrier and code tracking means 212 and 214 and to a data demodulator (not shown) which performs data demodulation to the signal.
FIG. 2C shows the spectrum shape of a wideband incoming signal Sin at an intermediate frequency fIF. FIG. 2D shows the spectrum shape of a signal obtained from the output of the carrier mixer 202 and down-converted to base frequency. FIG. 2E, in turn, shows the spectrum shape of a narrowband signal obtained from the output of the code mixer 204. However, FIGS. 2C to 2E are only intended to illustrate the shape of the spectrum of a signal, and not to present the actual spectrum of a signal.
The implementation of FIG. 2B is functionally identical to that of FIG. 2A. In this implementation, a local carrier replica, and a spreading code replica are combined in a mixer 213 to generate a local signal replica, and the incoming signal Sin is multiplied by this signal replica in a mixer 215. Otherwise, the signal processing corresponds to the implementation of FIG. 2A. This implementation is in use particularly in systems based on analog components, since it minimizes the number of components required on the signal path.
The implementation of FIG. 2A is widely used. The implementation of FIG. 2A is preferable to that of FIG. 2B, because spread spectrum receivers usually have to comprise several out-of-phase signal paths, starting from the multiplication by the spreading code replica, to enable the implementation of spreading code tracking. Spreading code tracking can be implemented for example with a correlator structure shown in FIG. 2F and comprising two out-of-phase signal paths 222 and 223, in which an incoming signal Scode freed from carrier modulation is correlated with an early C0 and late C1 spreading code replica generated locally with a code generator 224. A signal depending on the phase difference of the local spreading code replica and the code included in the signal Scode, is obtained from the output of an adder 226, and this signal is used to adjust the phase of the spreading code replica in the right direction. Spreading code tracking is typically carried out separately for I and Q signals, i.e. the number of required components is double compared with the structure of FIG. 2F.
A common feature in prior art implementations is that the carrier and the spreading code are removed at the same sampling frequency and that out-of-phase signal paths are processed in parallel.